This renewal proposal, like its predecessor, is concerned with the general topic of biological electron transfer. Specifically, it involves the synthesis of new, non-covalently assembled donor-acceptor model systems and the use of these to determine whether the details of pathway play an important role in regulating long-range electron transfer reactions. Three types of non-covalent pathway interactions have been chosen for study, namely hydrogen bonds, salt bridges, and van der Waals contacts. The importance of these putative mediating elements will be assessed by synthesizing and studying small molecular assemblies that contain Watson- Crick base pairs, electrostatic contacts, and nonbonded, cyclodextrin- derived receptor-substrate interactions, respectively, along the critical donor-to-acceptor electron transfer pathway. Well-studied species, such as photoexcited porphyrins, sapphyrins, N,N-dimethylaniline or in -situ - reduced porphyrins, will be used as the donors; likewise, viologens, quinones, electron-poor aromatics, and porphyrins will be used as acceptors. In all cases, the corresponding covalently linked systems will be prepared and studied as controls. The actual electron transfer events will be studied under both thermal and photoinduced conditions using a range of fast kinetic techniques, such as fluorescence quenching and time-resolved spectroscopy. From these studies, the rates of the individual electron transfer reactions will be deduced and the magnitude of the electron transfer matrix coupling term for each type of proposed bridging element derived. Congruent, quantitative comparisons will then be carried out and tile relative and absolute ET mediating efficacy of these three bridging interactions ascertained. Because of the quantitative nature of these comparisons, the present work is expected to provide a resolution to the current controversy surrounding the question of whether or not the details of protein pathway play a significant role in mediating the specific rates of long-range biological electron transfer reactions. This, in turn, is expected to further our understanding of these all-important processes. Biological electron transfer reactions are of fundamental and ubiquitous importance. They are crucial to a wide range of enzymatic processes and are intimately involved in both photosynthesis and oxidative phospho- rylation. The latter process is, of course, required for all higher life, including that of humans. As a result, either direct or indirect interference with the electron transport sequence of oxidative phosphorylation can have serious physiological consequences. For instance, azide and cyanide anions, which bind directly to the ferric form of cytochrome c oxidase are highly toxic. Moreover, factors which perturb heme synthesis, and hence reduce the efficiency of electron transport, also give rise to symptoms of long-term toxicity. Included in this category are certain hereditary diseases, such as acute intermittent porphyria, and chronic exposure to various heavy metal such as lead and arsenic. The proposed work, by furthering our understanding of biological electron transfer may ultimately lead to treatments for these induced or congenital disorders.